In other years, America's leading 3-year-old is asked to prove in the Kentucky Derby that he is a genuinely great racehorse. This year no one has such lofty expectations. The Derby favorite will be asked on Saturday to demonstrate that he can run in a reasonably straight line.

Marfa has emerged as the top colt in a generation devoid of stars, but some cynics have suggested that he should be competing in a rodeo instead of America's most famous horse race. After last week's Blue Grass Stakes, when he lugged in so badly that he prevented jockey Jorge Velasquez from riding him properly, interfered with two of his rivals and inflicted an injury on one of them, local writers dubbed him Marfa the Mugger.

"Marfa is a raw talent with a lot of ability, but he's still at a developmental stage," said Wayne Lukas, the colt's trainer. Lukas doesn't fully understand why Marfa seems bent on making a left-hand turn every time he enters the stretch, but he said, "The big thing is that Jorge understands. This horse respects discipline. I don't look for him to do those things if Jorge asserts himself."

If Marfa does behave on Saturday, he is the horse to beat in the 109th Kentucky Derby. Post time is 5:38 p.m. (WJLA-TV-7 starting at 4:30).

A late bloomer who had won nothing of consequence two months ago, he comes into the Derby with three straight strong performances. He captured the Jim Beam Spiral Stakes at Latonia by eight lengths and the Santa Anita Derby by three lengths before his antics in the stretch cost him a victory in the Blue Grass.

But if Marfa doesn't run his race, the 109th Derby is wide open. Although there are no superior 3-year-olds in the country this spring, there is a good deal of depth in this field. Of the 20 entrants, at least a dozen have a reasonable chance and four could be called prime contenders: Play Fellow, Slew O'Gold, Caveat and Sunny's Halo, the latter especially if the forecast for rain holds true. There is a 70 percent chance of thundershowers.

Freezing Rain, Balboa Native and Highland Park are also listed as "mudders."

Play Fellow has the breeding to win at 1 1/4 miles; he has the right running style; he has Jean Cruguet as his jockey. The first two times Play Fellow went a distance, Cruguet encountered a fair amount of trouble on the way. But in the Blue Grass he rode perfectly, made a strong move on the turn, sneaked through along the rail and scored a photo-finish victory over Marfa.

Seattle Slew has the chance to join the elite group of thoroughbreds who have both won the Derby and sired a Derby winner. His son, Slew O'Gold, captured the Wood Memorial Stakes at Aqueduct two weeks ago, and although that was the first stakes victory of his career, he appears to be maturing at the optimal time. His jockey is the incomparable Angel Cordero Jr.

Caveat is another colt who established himself as a Derby contender in his final prep race. In last Saturday's Derby Trial Stakes here, he rallied from last place to score an impressive victory. He has a Hall of Fame trainer, Woody Stephens, and a Hall of Fame jockey, Laffit Pincay Jr.; because of their credentials, Caveat and his stablemate Chumming may be second choice in the wagering.

Of all the 3-year-olds who have run in all the Derby prep races, none gave a more impressive performance than Sunny's Halo did in the Arkansas Derby. He ran 1 1/8 miles in 1:49 2/5 over the deep Oaklawn Park track, leaving the speed handicappers agog, and did it with an ease and authority that left the visually oriented handicappers agog.

Marfa's stablemate, Total Departure, is sure to set a fast pace. Desert Wine, another California colt, will be pressing him. Although trainer David Cross has taught Sunny's Halo to relax and sit behind the leaders, recent Derbies have shown that it can be fatal to be anywhere near a hot pace. Last year, when Gato del Sol won, the first three finishers were horses who had been running 19-18-16 after a quarter mile.

Several of the entrants have Maryland connections. Caveat was bred by Ryehill Farm in Mount Airy and could become the second Maryland-bred to win the race. Parfaitement is stabled at Bonita Farm in Bel Air and is trained by Billy Boniface, a prominent figure on the Maryland circuit. He finished a good second to Slew O'Gold in the Wood Memorial and could be a contender. Pax in Bello is owned by Mr. and Mrs. Arnold Willcox of Bethesda. Once highly regarded, his credentials were tarnished by his loss in the Derby Trial last week.

But it is California that dominates this race. Five of the horses in the field are based there, and if he doesn't misbehave, Marfa will be the second straight California horse to win the Derby.